1. Field of the Invention
The invention relates to signal trackers which predict the occurrence of a detected signal and, specifically, to digital signal trackers which predict the phase and pulse repetition intervals of a train of phase coherent signals while maintaining phase coherency with all signals provided at least one signal is present.
2. Description of the Prior Art
Formerly, signal trackers were analog devices which, while adequate for some purposes, were subject to noise interference and accuracy limitations so that they were not useful for applications requiring a certain high degree of precision. Moreover, these devices were comprised of a large number of components and therefore required a considerable amount of physical space as well as demanding a high power supply.
More recently, as described in the copending application of Kelly C. Overman, Ser. No. 608,305, now U.S. Pat. No. 4,040,054, signal trackers have been of the digital variety in which multiplexing techniques were utilized to reduce the physical space and operating power requirements of signal trackers. These digital trackers avoided the inaccuracy and environmental susceptibility of analog devices, thereby affording a new level of accuracy and dependability in tracking devices. However, each tracker in this multiplex group of prior art trackers could predict the phase and pulse repetition interval of each signal in the train being tracked only if at least half those signals were present. Specifically, memories within the tracker in which the pulse repetition intervals of the incoming signals were stored could accommodate the pulse repetition interval of only two signals. If the incoming signal were comprised of more than two signals, multiple pulse repetition interval memories were dedicated to each individual tracker. If, as in practice is often the case, both signals of a pulse repetition interval memory were lost for any extended period, the phase drift between the detected signals and the tracker's internal clock, coupled with errors in the pulse repetition interval prediction, would cause both signals to be lost and they would not be re-acquired until initial signal acquisition techniques were again instituted.
There was, therefore, a need for a digital signal tracker which would maintain phase coherency with a signal train comprised of a number of phase coherent signals having individual pulse repetition intervals without loss of signals which would go undetected for some considerable length of time and then reappear later in the signal train. Additionally, it was seen that it would be advantageous to have a signal tracker capable of tracking a signal train comprised of a high number of signals, but which would also conserve pulse repetition interval memory hardware.